The Global Positioning System (GPS) is an accurate, three-dimensional navigation system. GPS consists of a constellation of twenty one satellites and three spares that orbit the earth twice a day at an altitude of 10,898 miles, transmitting precise timing information. A network of ground stations and passive user receivers respond to the information. Each satellite continuously broadcasts pseudorandom codes at L-band frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. L1 is modulated with two types of code, the coarse/acquisition code (CA-code) and precision code (P-code). L2 carries an encrypted P-code. The network of ground stations are at precisely known locations. Transmissions are received from the satellites, analyzed and GPS time is compared with universal standard time at the ground stations. Corrections are transmitted to the satellites from the ground station. To determine a location (latitude, longitude and altitude) a user requires the simultaneous signals from four or more satellites orbiting the earth. Simultaneous signals from at least three satellites can be used to provide two dimensional positioning (latitude and longitude). The signals are analyzed and interpreted by the GPS receiver to determine the location. The interval between the transmission and the reception of the satellite signal is used to calculate the unit's distance from each of the satellites being used. Those distances are used in algorithms to compute a position.
A potential disadvantage of the world wide availability of the GPS satellites is the possibility that terrorists and hostile nations could use the navigational facilities to accurately aim weapons and to damage sensitive target areas from great distances.
Limiting the available accuracy of the GPS data reduces this threat and permits a wide variety of peaceful uses. Selective Availability (SA) is a method that reduces the accuracy for civilian and unauthorized users. SA inserts random errors into the system and reduces the CA-code accuracy. However, this prevents access by peaceful users to high precision navigational data. High precision navigational data is necessary for many applications, including the landing of commercial aircraft. The GPS system is well known and specified. See "The Journal of Navigation", volume 25 summer 1978, where the GPS system is described in detail.
Accordingly, there is a need to have high precision navigational data available for peaceful use while limiting access for hostile use.
Hostile users can steal, buy GPS receivers or build alternative receivers but lack the resources to create an entire equivalent or alternative navigational system. It is necessary to identify regions of the world, which may vary over time, where hostile actions are likely to occur.
High precision navigational data such as the encrypted GPS data, will be referred to as controlled access data. Trusted users that require access to high precision navigational data are provided with a plurality of receivers containing decryption means. This makes high precision positioning available to users that need it while preventing hostile users from having access. However the decryption means must be kept secured from hostile users. The level of security depends directly on the reliability of procedures and other safeguards for preventing the theft of the decryption means. The greater the number of mobile receivers equipped with decryption means the greater the likelihood that security will be breached. Thus a hostile user can obtain access to the high precision navigational data.
A hidden denial channel is another tactic to control access to high precision navigational data. It utilizes an encryption method that prevents a denial channel from being separated from the high precision navigational data. This encryption method prevents the denied data from being blocked without also blocking access to the encrypted high precision navigational data. The hidden denial channel can disable selected user sets. The denied users would be unable to defeat disablement indicative of denying access to the high precision navigational data. However, the hostile user can utilize a stolen decryption means to by-pass the restriction.
Another tactic for restricting user access to the high precision navigational data in hostile regions is to include data that identifies hostile regions in the encrypted portion of the signals. The receivers that have high precision navigational data decryption means would also include a function to inhibit access to the higher accuracy when the receiver is located within a hostile region. This will only protect the unaltered receiver. Unless the decryption means is made inseparable from the position calculation means, the security will not be effective. The hostile user can purchase or build an alternative GPS receiver. This receiver when coupled to the stolen decryption means can circumvent the inhibit function.
Thus, it is desirable to implement a system where stolen receivers with high precision navigational data decryption means are denied high precision access when located in hostile regions.
While a hostile region might also be denied access to the high precision navigational data by jamming methods, it is not obvious how this could be done without also denying access to friendly military users.
Another method uses a complex secret sharing algorithm which requires the reception of multiple keys from corresponding GPS satellites in order to gain access to the high precision navigational data Access to the high precision navigational data is denied within a denial region by not transmitting the corresponding key from the corresponding satellites that are visible within the denial region.
It is, therefore, an object of the present invention to provide an improved system to enable navigational receivers with selective access and denial based upon receiver location with respect to the reception of high precision navigational data.